22 C O N T E X T 1 4 4 : M A Y 2 0 1 6 St Mary de Castro Church, Leicester The guide book for this Grade I listed church describes the spire as ‘a constant source of trouble’, recording partial rebuilding in 1688, 1699, 1757, 1763, full rebuilding in 1783, further repairs in 1871, and the rebuilding of the top eight metres in 1916. Its condition had again deteriorated, severely, to the point that it was unsafe. A full grant was provided to allow it to be dismantled.The challenge was how to safely access the spire, in order to dismantle it.Various options (such as mobile crane and basket) were considered and discounted, mainly due to the restricted access into the courtyard in front of the church. The solution chosen was to scaffold the tower and spire. The issue with a scaffold of this height, over 60m, is that it could not practically be self supporting (that is, for it to include scaffold buttressing or guys), but had to be a ‘tied independent scaffold’, relying on the spire for its own lateral stability. But the spire could not be relied on to support the scaffold as it was unsafe. The factor of safety against collapse in high winds was considered to be close to unity, taking into account the extensive cracking and other deterioration that had taken place. Not only that, but adding a tied independent scaffold would significantly increase that lateral load on the spire. In detail, check calculations showed that in high wind conditions, the spire, weighing 75 tonnes, attracts a worst-case lateral wind load of around 13 tonnes.The scaffold in effect increases the surface area to resist the wind, passing a further 16 tonnes of wind loading on to the spire. In other words, the presence of the scaffold had the potential to more than double the wind loading on to the spire. Despite the fragility of the spire being made clear at the outset, the scaffold design drawing included a caveat stating that the client or their engineer was responsible to ensure that the structure could support the scaffold. A meeting was held with all stakeholders. This included building control, client, CDM principal designer, Historic England, principal contractor, scaffolding subcontractor and their scaffold designer. In this meeting we found a way forward whereby we all jointly acknowledged the risks, both of doing nothing, and of attempting to dismantle the spire.We jointly accepted the situation that the spire could not be relied on to support the scaffold in high wind, and crucially accepted that we considered the risk to the general public of doing nothing outweighed that of proceeding. Scaffolding commenced and mitigation measures were implemented to reduce the risk to the operatives and public. Roads were closed, nearby buildings remained vacated, wind-speed predications were obtained on a daily basis from the Met Office, and a system of live high-level wind-speed monitoring was used, so that the scaffold and surrounding area could be vacated if the wind speed became too great. Strapping was employed across the crack lines, and cracks were infilled as scaffolding proceeded to improve the spire’s robustness. Despite the calculated high lateral loads from the scaffold, it was recognised that the scaffold would have held an inherent strength and stiffness from its structural form, as it cantilevered up from its root, wrapped around the tower below the spire. Hidden strength such as this is not taken into consideration in the standard calculations or guidance used to determine the loading on to the structure, and it is likely that this hidden strength contributed to the successful outcome of the project. It was quite an experience standing on top of the freshly completed scaffold, just before dismantling commenced, and to feel the sway in mild wind of the scaffold in union with the spire. John Ruddy, a conservation accredited structural engineer, is director of Capstone Consulting Engineers. At St Mary de Castro the scaffold depended on the spire for support – but the spire itself was unsafe.
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